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Communication Interfaces 10-20
Serial Interface Busy-Handling Protocols
The serial interface provides several methods for the printer to
indicate its ready and busy conditions. The communication interface
menu, described in Chapter 2 of this manual, allows you to select
any of these methods during printer setup. The following paragraphs
describe the serial interface busy-handling protocols.
Flow-Control Busy-Handling Protocols
These protocols are busy-handling protocols determined by the host
that can be selected via the printer setup menu. The menu structure
allows you to select either of the following flow-control busy-handling
protocols or to set both protocols inactive.
q Transmit-On/Transmit-Off Protocol — When you select this
busy-handling protocol, the printer sends a transmit-off (X-off)
signal to indicate that its receive buffer is full or some other busy
condition exists. When the printer is again ready to receive data,
it sends a transmit-on (X-on) signal.
These signals (X-on and X-off) are equivalent to the device
control characters DC1 and DCS, respectively, of the ASCII
character set. Thus, the printer sends the DC3 control character
to indicate that it is busy, and the DC1 control character to
indicate that it is ready to receive data.
q End-of-Text/Acknowledge Protocol — When you select this
busy-handling protocol, the host sends data in blocks of 256
characters or less. At the end of each block transfer operation,
the host sends the end-of-text (ETX) control character to indicate
that the operation is complete. The printer responds by sending
the acknowledge (ACK) control character to indicate that it is
ready to receive any subsequent data.
If the receive buffer is full (or some other busy condition exists),
the printer delays sending the ACK control character until the
busy condition is cleared.